Rear-mounted gimbal for supporting test head

ABSTRACT

A manipulator for supporting heavy as well as lighter test heads in a small footprint includes a body and an interface for supporting a test head from behind. The interface includes a first portion fixedly attached to the body and a second portion fixedly attached to the rear of the test head. The first and second portions of the interface are rotatably coupled together to allow rotation of the test head about its approximate center of mass. Although the weight of the test head is entirely borne from the rear, the test head can still be moved with relatively little applied force, thereby satisfying the requirements for compliant docking.

This application claims the benefit of Provisional application Ser. No60/359,863, filed Feb. 27, 2002.

CROSS-REFERENCES TO RELATED APPLICATIONS

Not Applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable.

REFERENCE TO MICROFICHE APPENDIX

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to precisely positioning heavy objects,and more particularly to precisely positioning a test head of anelectronic automatic test system for docking the test head with aprober, handler, or other peripheral for testing electronic devices.

2. Description of Related Art

Manufacturers of semiconductor chips and assemblies use automatic testequipment (“ATE”) to verify the performance of devices before thedevices are shipped to customers. ATE systems typically include a “testhead” and a “tester body.” The test head houses portions of the testsystem that are preferably located as close as possible to the deviceunder test, and connects to the tester body via one or more cables. Fortesting electronic devices, the test head connects or “docks” with aperipheral. The peripheral feeds a series of devices to the ATE systemfor testing, and the ATE system tests the devices.

Constraints affecting semiconductor test processes often make itimpractical to move the peripheral to the test head. In mostmanufacturing facilities, therefore, the peripheral that feeds the chipsremains stationary, and the test head is moved into position for dockingwith the peripheral.

A device called a “manipulator” is used to move the test head to theperipheral. As is known, a common type of manipulator is the fork-armmanipulator, an example of which is shown in FIG. 1. Here, a manipulator100 holds a test head 112 from its sides via fork arms 114. Themanipulator 100 raises and lowers the test head 112 on a linear stage118, and rotates the test head about a twist axis upon a twist gear 120.A fork arm manipulator like the one shown in FIG. 1 is disclosed in U.S.Pat. No. 5,949,002, entitled, “Manipulator for Automatic Test Equipmentwith Active Compliance,” which is assigned to Teradyne, Inc., of Boston,Mass., and is hereby incorporated by reference.

FIG. 2 shows another type of manipulator used for positioning a testhead. Rather than using fork arms, the manipulator 200 of FIG. 2supports a test head 210 internally. The test head 210 is mounted to acentral stiffener 212, which is, in turn, mechanically coupled to themanipulator via a central blade (not shown), which extends approximatelythrough the center of the test head. The manipulator 200 can raise andlower the test head 210 on linear bearings 224, and can rotate the testhead in the twist direction via twist bearing 214. It can also swing thetest head 210 via a swing bearing 222. A manipulator like the one shownin FIG. 2 is disclosed in U.S. patent application Ser. No. 09/615,292,entitled “AUTOMATIC TEST MANIPULATOR WITH SUPPORT INTERNAL TO TESTHEAD.” This application is also assigned to Teradyne, Inc. and is herebyincorporated by reference.

Both types of manipulators generally include actuators such as motors(not shown) on their respective bearings and linear stages. Theactuators move the test head to the peripheral, and orient the test headfor docking. The test head is then docked with the peripheral by finelyadjusting the position and orientation of the test head.

Manipulators commonly provide a range of “compliance” that allows a testhead to be rotated about one or more axes as the test head andperipheral are being docked. The range is “compliant” because the testhead literally complies with forces applied to the test head, whichduring docking tend to cause the mating surface of the test head tobecome coplanar with the mating surface of the peripheral.

In the fork arm manipulator of FIG. 1, the test head 112 can be made tonod compliantly in a “tumble” direction (NU and ND) by rotating the testhead about pins 116. One pin 116 is provided within each fork arm oneach side of the test head 112. The test head can also be made to turncompliantly in a “theta” direction (ΘL and ΘR), via the movement ofmechanisms within each fork arm, which allow the pins 116 to moveslightly back and forth along the length of each fork arm. Opposingmovements of the pins 116 on opposite fork arms effects compliant thetarotation.

The test head 210 of FIG. 2 can compliantly rotate in theta, tumble, andtwist via a spherical bearing (not shown) positioned approximately atthe test head's center of mass. The spherical bearing has an inner racecoupled to the central stiffener 212 and an outer race coupled, via thecentral blade, to the twist bearing 214.

Semiconductor manufacturers and semiconductor testing facilities place ahigh premium on minimizing the floor space that their ATE systemsoccupy. As semiconductor devices become more complex, however, the ATEsystems used to test them tend to become larger, requiring more floorspace. We have recognized that relatively small test heads (e.g., lessthan 200 kg) can be held effectively with fork arms that do not occupymuch additional floor space. For larger test heads, however, fork armsare required to grow substantially in size, to the point where theyoccupy a significant percentage of the ATE's overall footprint.

The internally supporting manipulator, like the one shown in FIG. 2,tends to hold heavier test heads with less floor space than anequivalent fork arm manipulator would require. ATE systems now includetest heads weighing over 1300 kg. For these larger test heads,internally supporting manipulators are generally the more spaceefficient alternative.

We have recognized that the centrally supporting manipulator places manyconstraints on the design of the test head that it supports. Forinstance, the test head generally must be provided in two portions 210 aand 210 b, which independently attach to the central stiffener 212.Electrical cables connecting the two portions must be passed eitherthrough the central stiffener or around it. Because the centralstiffener and associated hardware occupy the center of the test head210, this area is not available for other purposes, such as cooling andadditional electronics.

What would be desirable is a manipulator that is more space efficientfor heavy test heads than the fork-arm manipulator but does not imposethe design constraints associated with the centrally supportingmanipulator.

BRIEF SUMMARY OF THE INVENTION

In accordance with the present invention, a manipulator for supporting atest head includes a body and an interface for supporting the test headfrom one of its faces, for example, from the rear. The interfaceincludes a first portion fixedly attached to the body and a secondportion fixedly attached to the test head at one of its faces. The firstand second portions of the interface are rotatably coupled together toallow rotation of the test head about its approximate center of mass.Because the test head is rotatable about its center of mass, it can bemade to rotate in response to relatively small applied forces, tosatisfy the requirements of compliant docking.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional objects, advantages, and novel features of the invention willbecome apparent from a consideration of the ensuing description anddrawings, in which—

FIG. 1 is a perspective view of a fork arm manipulator according to theprior art;

FIG. 2 is a perspective view of a manipulator for internally supportinga test head according to the prior art;

FIGS. 3A-3C are a series of front, elevation views of a manipulatoraccording to a first embodiment of the invention, which shows the testhead rotated to different orientations;

FIG. 4 is an exploded, perspective view of an interface assembly usedfor rotatably coupling the manipulator to the test head in thearrangement of FIGS. 3A-3C;

FIG. 5 is an elevated, perspective view of the interface assembly ofFIG. 4;

FIG. 6 is an elevated, front view of the interface assembly of FIGS. 4and 5;

FIG. 7 is an elevated, perspective view of a portion of an interfaceassembly and test head according to a second embodiment of theinvention; and

FIGS. 8A and 8B are elevated, perspective views of the embodiment ofFIG. 7, which show the test head rotated to different orientations.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIGS. 3A-3C show different views of a manipulator 310 for positioning atest head 312 according to a first embodiment of the invention. A deviceinterface board (DIB) 316 is attached to the test head 312 forinterfacing the test head with a peripheral (not shown), such as aprober or a handler. The manipulator 310 includes a body 310 a mountedto a base 310 c. An arm 310 b extends from the body 310 for supportingthe test head 312. The manipulator 310 can move the arm 310 b up anddown, in a Z-direction, for positioning the test head vertically inspace.

An interface assembly 314 connects the test head 312 to the manipulator310. The interface assembly includes a first portion attached to thebody 310 a of the manipulator (via the arm 310 b) and a second portionattached to the test head 312. The first and second portions are coupledtogether in a manner that supports the weight of the test head, yetallows the test head to be rotated about any of the conventionalrotational components: theta, tumble, and twist.

FIG. 3A shows the test head 312 rotated to the left in theta, but withtumble and twist nominally at zero degrees. In FIG. 3B, theta and twistare at zero degrees but the test head is rotated downwardly in tumble.In FIG. 3C, theta is at zero degrees, but the test head is rotatedupwardly in tumble and downwardly in twist. It should be appreciatedthat the dimensions and proportions of the equipment shown in FIGS.3A-3C are presented for illustrative purposes and are not necessarily toscale.

From these views, it is apparent that the first portion of the interfaceremains in a fixed position with respect to the manipulator and thesecond portion of the interface remains in a fixed position with respectto the test head. The first and second portions of the interface movewith respect to each other, however, to accomplish rotation of the testhead.

Although it may not be immediately apparent, the relative movement ofthe first and second portions of the interface is not mere rotation,such as that accomplished by a hinge. Rather, the second portion of theinterface moves in arc-shaped paths with respect to the first portion ofthe interface. By geometric projection, the arc-shaped paths can be seento have a common center located within the body of the test head,approximately (and ideally) at its center of mass.

Because the center of the test head's rotation is its center of mass(approximately), the test head rotates about its center of mass, as ifit were being held from this point, even though the test head is beingsupported entirely from the rear. This means that the test head iseasily moveable in compliance with normal docking forces. Docking forcesneed not be sufficient to lift the weight of the test head; they needonly be sufficient to overcome friction and small imbalances that ariseif the geometric center of rotation does not precisely coincide with thetest head's center of mass.

An illustrative embodiment of the interface 314 can be more clearly seenin the exploded view of FIG. 4, and in the perspective side view andfront view of FIGS. 5 and 6. The first portion, i.e., the part thatnormally attaches to the manipulator, includes a support plate 410 and apair of substantially U-shaped brackets 412. The U-shaped brackets arefixedly attached to an upper region of the support plate 410 by anyappropriate means, such as screws, bolts, welding, brazing, and soforth. Low friction contacts 414, such as ball casters (also known asball transfers), are held by the U-shaped brackets in a position thatenables their contact regions, such as balls 414 a, face to the rear, inthe direction of the support plate. In particular, the ball casters 414include a cylindrical back portion that is inserted into a hole 412 a ineach U-shaped bracket, from the inside of the respective U-shapedbracket. The ball casters are then fastened to the insides of theU-shaped brackets, for example, using bolts, although other types offastening can be used.

A pair of standoff blocks 418 is fastened to a lower region of thesupport plate 410. Each standoff block 418 preferably includes a hole418 a for receiving an additional low friction contact, such as a ballcaster 420. The ball casters 420 are preferably of the same type as theball casters 414 used at the upper region of the support plate 410. Thestandoff blocks 418 and the U-shaped brackets 412 hold their respectivelow friction contacts at respective angles, which roughly conform to theplane of the surfaces with which they make contact, i.e., those of thedish-shaped section 440.

The dish-shaped section 440 forms part of the second portion of theinterface, i.e., the portion that is attached to the test head. Thedish-shaped section preferably has a central aperture 440 a, an concaveinner surface facing the test head, and a convex outer surface facingthe first portion of the interface. The inner and outer surfaces areeach preferably spherical in shape (i.e., they are each sections of asphere), and the spherical surfaces are preferably concentric. Thedish-shaped section fits into the U-shaped brackets 412 such that itsinner surface makes contact with the balls 414 a of the ball casters 414and its outer surface makes contact with the balls 420 a of the ballcasters 420 (best seen in FIG. 5). The dish-shaped section 440 ispreferably attached to the test head via a cylindrical section 450. Thedish-shaped section 440 is preferably bolted to the cylindrical section450, which is preferably bolted in turn to the test head. As is usuallythe case here, the exact fastening technology is not critical, providedit is sufficient to support the expected loading with adequate safetymargin.

The role of the cylindrical section 450 is to provide spacing betweenthe dish-shaped section 440 and the test head to which it attaches. Thisspacing is needed to prevent the U-shaped brackets 412 and standoffblocks 418 from making contact with the rear of the test head as thefirst and second portions of the interface are moved over their fullallowable range. Rather than providing the dish-shaped section 440 andthe cylindrical section 450 as separate pieces, they could instead beprovided as a single, continuous piece. Alternatively, the cylindricalsection 450 could be replaced with separate standoffs that attach aroundthe outer circumference of the dish-shaped section 440, for maintainingthe requisite spacing. If this variation is adopted, care should betaken, however, to prevent users from pinching body parts between thedish-shaped section and low friction contacts.

To limit the range of upward tumble rotation, the U-shaped brackets 412preferably include bumper pins 416. The bumper pins 416 preferably eachhave a central spring-loaded pin that engages holes 412 b in eachU-shaped bracket, for holding them near the inside bottom of therespective U-shaped stiffener. They also each have an outer rubber tubesurrounding the spring-loaded pin. The rubber tubes prevent damage tothe dish-shaped section 440 by providing cushioned contact between thedish-shaped section 440 and the U-shaped brackets 412 when the test headis rotated to its maximum tumble angle. Other cushioned stops (notshown) are preferably also used, to provide cushioned contact at minimumtumble angle, as well as at maximum and minimum theta angles. Theseother cushioned stops can be implemented in a variety of ways, orinstance, by attaching cushioned right angle brackets at appropriatepositions and orientations on the support plate 410.

As indicated in the figures, the dish-shaped section 440 is held inplace at four contact points (i.e., two instances at 414 a and twoinstances at 418 a). To prevent the dish-shaped section 440 from ridingon only three of these contact points, the ball casters 414 and 420 arepreferably spring-loaded. All four ball casters thus share the loadinduced by the test head approximately equally, in spite of minortolerance errors.

Because the inner and outer surfaces of the dish-shaped section 440 arepreferably spherical and concentric, and because the centers of thesespherical surfaces approximately intersect the test head's center ofmass, the forces between the dish-shaped section and each of the lowfriction contacts are almost entirely normal. This means that, with thetest head in place, there is substantially no rotational moment betweenthe first and second portions of the interface. The interface thereforeremains stable and does not tend to fall or tilt under the weight of thetest head. The forces needed to rotate the test head are only thoseneeded to overcome friction and loading imbalances.

It is apparent from the foregoing that the interface 314 allows the testhead to be compliantly rotated about its center of mass in tumble (upand down). Since the dish-shaped section 440 is spherical, it shouldalso be apparent that the test head can be compliantly rotated about itscenter of mass in theta (left and right). In addition, since thedish-shaped section 440 forms a complete ring, it should further beapparent that the test head can be rotated about its center of mass intwist (clockwise and counterclockwise).

This design has other advantages. Because it is open in the center,electrical cables emanating from the test head can be passed through thecenter of the interface back to the manipulator, through a hole 410 a inthe support plate 410. This keeps the cables out of sight and causestheir weight to induce a minimum load imbalance. The central region islarge enough to allow other components, such as air ducts, blowers, orother cooling equipment, to be positioned therein for cooling theelectronics within the test head in a highly space-efficient manner.

The interface 314 and all of its parts are preferably steel, althoughother metals or materials can be used that meet the loading requirementswith adequate safety margin. The dish-shaped section 440 is preferablyhardened steel. Hardened steel is preferred because it minimizesdistortion in the shape of the dish-shaped section 440, particularlyunder the load of heavy test heads. In the preferred embodiment, thedish-shaped section is approximately 0.6 m in diameter and approximately1.5 cm thick. In general, the smaller the diameter of the dish-shapedsection 440, the greater the thrust forces between it and thelow-friction contacts. Therefore, decreasing the diameter of thedish-shaped section 440 generally requires increasing its thickness andincreasing the thrust ratings of the low-friction contacts.

The interface 314, and a manipulator constructed therewith, is suitablefor supporting both exceedingly heavy and relatively light test heads.For supporting heavy test heads, it avoids the need for large fork arms,which require considerable space. It also avoids the need for internallysupporting the test head, which places many constraints on the testhead's design.

Alternatives

Having described one embodiment, numerous alternative embodiments orvariations can be made. For example, in the above-described embodiment,the dish-shaped section 440 has been shown and described as part of thesecond portion of the interface 314 (i.e., attached to the test head),and the low friction contacts have been shown and described as part ofthe first portion of the interface 314 (i.e., attached to themanipulator). This could be reversed, however, by attaching thedish-shaped section 440 to the manipulator and the low friction contactsto the test head.

As shown and described, the U-shaped brackets 412 reach to the innersurface of the disk-shaped section 440 through its central aperture 440a. Alternatively, the U-shaped brackets could be made to reach aroundthe outside of the disk-shaped section 440. The cylindrical section 450could then be reduced in size and made to attach near the inner diameterof the dish-shaped section. This variation would allow the aperture 440a to be “removed” from the disk-shaped section 440, leaving it more inthe shape of a “cap” than a “ring.”

As shown and described herein, the disk-shaped section 440 forms acomplete ring, which allows the test head to be rotated in twist over anarbitrarily large angle. Alternatively, the disk-shaped section 440could be constructed as a partial ring, such as a horseshoe, forproviding a smaller range of twist rotation. As yet another variation,the disk-shaped section 440 could be provided as a pair of opposingsections, for instance, left and right or top and bottom sections, withthe other portions of the ring removed. The size of the sections couldbe made large enough to allow small twist rotations, such as thoseneeded for compliant docking, but would not necessarily be large enoughto allow gross adjustments of the test head's twist angle.

Depending on the desired performance, the inner and outer surfaces ofthe disk-shaped section 440 need not be perfectly spherical. Forexample, slightly opening the curvature of the outer surface, closingthe curvature of the inner surface, or both, induces a rotational momentthat tends to restore the test head to a centered position, once it hasbeen moved from its centered position.

The low friction contacts 414 and 420 have been particularly describedherein as thrust ball caster assemblies; however, other types ofcontacts can be used, such as other types of rolling contacts orfrictional contacts. An example of a “frictional” low-friction contactis Teflon pads. Teflon or other low friction plastics or materials maybe used instead of roller bearing assemblies, particularly inlightweight, low cost test systems. “Frictional” low friction contactsmay be better used against some surfaces than others. Therefore, if thisvariation is adopted, it may be advantageous to cover the dish-shapedsection 440 with a surfacing material having a particularly lowcoefficient of friction when used in connection with the “frictional”low friction contacts. For some test heads, it may be advantageous touse air bearings in place of conventional roller bearings to reducefriction to negligible levels. Suitable air bearings are availablecommercially from Space Electronics, Inc., of Berlin, Conn.

As describe above, four bearing assemblies support the test head via thedish-shaped section 440. The number of bearing assemblies could clearlybe varied within the scope of the invention. In addition, the contactsneed not be identical. For instance, a pair of thrust bearings could beused at the top and bottom of the support plate 410 for supporting theweight of the test head, and a pair of “frictional” contacts could beused at the left and right of the support plate for maintaining itsposition and guiding its theta rotation.

The interface 314 need not be provided as a separate unit, to be laterintegrated with a manipulator and a test head. Instead, one portioncould be built into the manipulator and another portion could be builtinto the test head. The parts of the interface could then be matedtogether when the test head is integrated with the manipulator.

The interface 314 can be varied in a more basic manner within the scopeof the invention. For example, as shown in FIGS. 7-8B, a secondembodiment of art interface for supporting a test head providescompliant rotation by breaking down the test head's rotation, intoseparate components of theta and tumble. This interface preferablyprovides four channels 718 for allowing rotation in theta and fourchannels 722 for allowing rotation in tumble. Cam followers preferablyride within the arc-shaped channels to allow relative rotation ofdifferent parts of the interface. For each component of rotation, therespective channels share a common axis of rotation (i.e., they arealigned concentrically), which approximately (and Ideally) intersectsthe test head's center of mass. Because the axis of rotation for eachcomponent intersects the center of mass, the test head has no rotationalmoment, and thus tends not to fail or tilt, even though it is heldentirely from the rear.

A manipulator supports the interface by attaching to a first framesection 712. Cam followers 716 preferably project upwardly anddownwardly from corners of the first frame section 712, for engagingchannels 718, which are preferably formed within a second frame section714. Movement of the first frame section 712 with respect to the secondframe section 714 thus effects theta rotation of the test head (See FIG.8A).

To effect tumble rotation, tabs 714 b are fixedly attached to and extendfrom the rear of second frame section 714 (from the viewpoint of FIG.7). Cam followers 720 extend from these tabs 714 b for engaging channels722, which are preferably formed within tabs 710 a of the test headframe 710. Movement of the second frame section 714 with respect to thetest head thus accomplishes the desired tumble rotation of the test head(See FIG. 8B).

This second embodiment of the interface is analogous to the firstembodiment. The first and second frame sections 712 and 714 and theirassociated hardware constitute a first portion of the interface (i.e.,one to be attached to the manipulator), and the tabs 710 a extendingfrom the test head constitute the second portion of the interface. Thefirst and second portions move relative to each other to allow compliantrotation of the test head approximately about its center of mass. Unlikethe first embodiment, however, this embodiment does not supportcompliant rotation in twist. If needed, compliant twist rotation can beaccomplished though the use of separate equipment.

Where twist rotation is externally provided, it is preferable to preventthe tabs that extend from the second frame section 714 b from rubbingagainst the tab sections 710 a. This can be accomplished by placinglow-friction pads, ball casters, or similar components, between the tabs714 b and 710 a.

To limit the speed at which the test head can be rotated, the interfaceof this embodiment preferably includes rate limiters, such as gassprings 724. A first pair of gas springs is preferably coupled betweenthe first and second frame sections 712 and 714, and a second pair ofgas springs is preferably coupled between the second frame section 714and the test head.

The second embodiment of the interface can be varied in numerous ways,many of which are similar to those described in connection with thefirst embodiment, and will not be repeated here. Other variationsinclude replacing the channels 718 and 722 with tracks, for example,curved linear bearings. In addition, the relative positions of thechannels and cam followers can be exchanged. For instance, channels canbe formed within the first frame section 712 (or a tab extendingtherefrom) for engaging cams extending from the second frame section714.

The channels can be made arbitrarily long or short, depending upon howmuch rotation is desired, whether gross or merely fine, compliantadjustment is required, and other design considerations.

The shape of the channels (or tracks) can depart from perfectly circulararcs. By opening the curvature of the channels, the test head will besubjected to a restoring force that tends to drive it toward a centeredposition.

Although it provides compliant rotation in both theta and tumble, theinterface of FIGS. 7 and 8 could be simplified to provide rotation inone direction only, i.e., either theta or tumble. For example, byremoving the first frame section 712 and attaching the second framesection 714 directly to the manipulator, the interface can be made toprovide tumble rotation only. Similarly, by attaching the second framesection 714 directly to the test head, the interface can be made toprovide theta rotation only.

As used herein, the “rear” of the test head conventionally refers to theface of the test head that attaches to the manipulator. This designationis clearly arbitrary, as the test head could attach to the manipulatorat a number of its different faces. Also, the interface 314 and thecorresponding interface of the second embodiment each have a secondportion attached to the rear of the test head. However, the particularmeans of attaching to the rear of the test head need not be accomplishedvia connections at the rear surface of the test head. For instance, thesecond portion of the interface could be connected to the rear of thetest head via brackets or other hardware extending from its sides, top,bottom, or inside. Moreover, the second portion of the interface can be“attached” to the test head by being formed coextensively with the testhead, rather than being a separate part. This variation is used in thesecond embodiment, wherein the tabs 710 a are actually formed within theframe 710 of the test head.

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and detail may bemade therein without departing from the spirit and scope of theinvention.

What is claimed is:
 1. A manipulator for an automatic test system,comprising: a body; and an interface for supporting a test head, theinterface including a first portion coupled to the body and a secondportion fixedly attached to a rear surface of the test head, one of thefirst and second portions of the interface including a substantiallydish-shaped section, and the other of the first and second portionsincluding a plurality of low-friction contacts attached thereto andcoupled to the dish-shaped section for allowing rotation of the testhead about its approximate center of mass in at least one of a theta andtumble direction.
 2. A manipulator as recited in claim 1, wherein theplurality of low-friction contacts comprises a plurality of thrustbearing assemblies.
 3. A manipulator as recited in claim 1, wherein thedish-shaped section has an inner surface and an outer surface that aresubstantially spherical and substantially concentric.
 4. A manipulatoras recited in claim 3, wherein the plurality of low-friction contactsincludes at least one low-friction bearing assembly coupled to an upperregion of the inner surface of the dish-shaped section and at least onelow-friction assembly coupled to a lower region of the outer surface ofthe dish-shaped section.
 5. A manipulator as recited in claim 3, whereinthe dish-shaped assembly includes an aperture for conveying electricalcables between the test head and other portions of the automatic testsystem.
 6. A manipulator as recited in claim 3, wherein the interfacefurther comprises: a support plate; and at least one substantiallyU-shaped bracket fixedly attached to an upper region of the supportplate, wherein at least one of the plurality of low-friction contacts isfixedly attached to the at least one substantially U-shaped bracket andpositioned in contact with the inner surface of the dish-shaped section.7. A manipulator for an automatic test system comprising: a body; and aninterface for supporting a test head, including a first portion coupledto the body and a second portion adapted for fixed attachment to a rearsurface of the test head, one of the first and second portions of theinterface including a plurality of partial dish-shaped sections, and theother of the first and second portions including a plurality oflow-friction contacts attached thereto and coupled to the plurality ofpartial dish-shaped sections for allowing rotation of the first portionof the interface with respect to the second portion of the interface. 8.A manipulator for an automatic test system, comprising: a body; and aninterface for supporting a test head, the interface including a firstportion coupled to the body and a second portion adapted for fixedattachment to a rear surface of the test head, wherein the first andsecond portions of the interface are coupled together via a plurality oflow-friction contacts constrained to move along arc-shaped paths toallow rotation of the test head about its approximate center of mass inat least one of a theta and tumble direction.
 9. A manipulator asrecited in claim 8, wherein the low-friction contacts are cam followersand the arc-shaped paths are defined by channels within which the camfollowers can move.
 10. A manipulator as recited in claim 8, whereinfour arc-shaped paths are provided for rotation about a first axis ofrotation, the four arc-shaped paths being provided in two pairs, eachpair having a common center.
 11. A manipulator as recited in claim 10,wherein the first axis is one of a theta and a tumble axis.
 12. Amanipulator as recited in claim 10, wherein four additional arc-shapedpaths are provided for rotation about a second axis of rotation, thefour additional arc-shaped paths being provided in two pairs, each pairhaving a common center.
 13. A manipulator as recited in claim 12,wherein the first axis is one of a theta and a tumble axis, and thesecond axis is the other of the theta and the tumble axis.
 14. Amanipulator as recited in claim 8, wherein the first portion of theinterface includes first and second frame sections, a first framesection fixedly attached to the body of the manipulator and rotatablyattached to the second frame section to allow rotation of the test headabout a first axis, and a second section rotatably attached to thesecond portion of the interface to allow rotation of the test head abouta second axis.
 15. An interface for attaching a manipulator to a testhead in an automatic test system, comprising: a first interface portionfor attaching to the manipulator; and a second interface portion forfixedly attaching to a rear surface of the test head, wherein the firstand second interface portions are moveably coupled together to allowrelative movement along arc-shaped paths having centers thatsubstantially intersect an expected center of mass of the test head,said relative movement effecting rotation of the test head in at leastone of a theta and a tumble direction.
 16. An interface as recited inclaim 15, wherein one of the first and second interface portionsincludes a substantially dish-shaped section, and the other of the firstand second interface portions includes a plurality of low-frictioncontacts attached thereto and coupled to the dish-shaped section forallowing rotation of the first interface portion with respect to thesecond interface portion.
 17. A manipulator as recited in claim 15,wherein the first and second interface portions are coupled together viaa plurality of low-friction contacts constrained to move alongarc-shaped paths.
 18. A manipulator as recited in claim 17, wherein thelow-friction contacts are cam followers and the arc-shaped paths aredefined by channels within which the cam followers can move.